1. Field of Invention
This invention relates to control apparatus for heating, ventilating, and air conditioning (HVAC) systems, and more particularly to control of terminal reheat type HVAC systems to reduce actual energy consumption required for maintaining desired environmental temperature conditions.
2. Description of the Prior Art
As described in the hereinbefore reference copending application, HVAC systems are used for controlling the environmental temperature conditions in one, or a plurality of structurally enclosed living spaces, i.e. those spaces inhabited by humans. In a terminal reheat type of HVAC system, of a type known in the art, the system provides temperature regulation in one or more living spaces by providing conditioned air to each of the spaces which is controlled to a determined cool discharge temperature value in an air handler source. The source may typically include a cooling source to provide precooled air at a determined discharge temperature to each of the plurality of spaces. Each space includes a terminal heat source located within each of the respective spaces. A system of this type is shown and described in U.S. Pat. No. 3,951,205 issued on Apr. 20, 1976 to A. H. Zilbermann, where a central utility means including a cold air source (generally referred to as an air handler source) presents cold discharge air through a main supply duct to each of a plurality of living spaces. The cold discharge air is received through a distribution unit in each space, and includes inlet and outlet dampers, a blower or supply fan, and a heating element. The cold discharge air is presented through the distribution unit into the space, and the temperature within the space is sensed by the thermostat. The heating element is positioned in the outlet duct of the distribution unit and is responsive, together with the supply fan and damper assemblies, to a pneumatic control signal presented from a thermostat within the space.
The inlet damper position is modulated in response to the pneumatic signal from the thermostat, i.e. too high a temperature drives the damper towards the full open position. For decreasing temperatures within the space the inlet damper is driven towards the full closed position, and if the temperature continues to decrease the heating element is actuated to provide a reheat of the cool air driven through the distribution unit. The pneumatic control loops for the space distribution unit are proportional gain loops. As described in the hereinbefore reference copending application, and as may be known to those skilled in the art, a proportional gain loop requires a constant output error term, i.e. a constant space temperature error to maintain a pneumatic signal of sufficient magnitude to drive the space dampers and reheat coils. The required space temperature error is referred to as a "droop error" and the magnitude of the "droop error" is dependent on the magnitude of the proportional gain selected, i.e. lower values of proportional gain result in a large "droop error" value, and higher values of proportional gain result in excessive limit cycle amplitudes due to the inherent time lags and thermal response of the system. These inherent time lags in system response result in a race condition, or "fighting", of the cold air source and the individual space reheat coil. To illustrate, a condition where the spaces are too cold results first in a closing of the inlet dampers, which in turn reduces the cold source load and causes a decrease in the cold air discharge temperature from the cold source. The cold source control loop requests a higher discharge temperature while at the same time, the individual space reheat sources are energized to increase the space temperatures. The overshoot of the space temperature then results in a decrease or shutoff of the reheat coils concurrent with an opening of the inlet dampers. The opened dampers increases the cold source load, increasing the cold source discharge temperature and requiring the cold source loop to increase the flow of the cold source medium, i.e. chill water, resulting in increased chill water consumption. This limit cycling of the cold source discharge temperature, i.e. typical peak-to-peak limit cycle amplitudes in cold deck temperature of 20.degree. are common, consumes excessive amounts of energy in addition to the obvious waste in control energy in cycling the heating or cooling sources through these temperature extremes.